The present invention relates, in general, to memory devices and, more particularly, to nonvolatile memory devices.
Electrically Erasable Programmable Read Only Memory (EEPROM) permits storing data that can be retained even if power to the memory is removed. An EEPROM cell stores data by storing electrical charge in an electrically isolated floating gate of a Field Effect Transistor (FET). The stored charge controls the threshold of the FET, thereby controlling the memory state of the EEPROM cell. Two common structures for an EEPROM cell are a double poly structure and a single poly structure. The double poly structure is conventionally fabricated using two polycrystalline silicon layers. The single poly structure requires only one polycrystalline silicon layer and, therefore, is simpler and less expensive to fabricate than the double poly structure.
In a single poly nonvolatile memory cell, an isolated region in the silicon substrate controls the memory state of the memory cell. A floating polycrystalline silicon gate covers both the gate oxide layer of the FET and a dielectric layer above the isolated region. Conventionally, the isolated region is a well of one conductivity type, e.g., N conductivity type, in the silicon substrate of a complementary conductivity type, e.g., P conductivity type. Because the P-N junction between the well and the substrate should be reverse biased to provide isolation between the well and the substrate, a well in the substrate can be used to either program or erase the memory cell. Thus, a single poly novolatile memory device having a single well is usually either electrically erasable or electrically programmable, but not both. A conventional single poly EEPROM cell includes two wells in the substrate, one for electrically programming the memory cell and the other for electrically erasing the memory cell. Therefore, the conventional single poly EEPROM cell requires a large silicon area.
Accordingly, it would be advantageous to have a nonvolatile memory device and a method for accessing the nonvolatile memory device. It is desirable for the nonvolatile memory device to be simple and inexpensive to fabricate. It is also desirable for the nonvolatile memory device to be silicon area efficient. It would be of further advantage for the nonvolatile memory device to be both electrically programmable and electrically erasable.